igneous
Basalt (Water-worn Pebble)
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-glassy; Crystal Structure: Aphanitic (fine-grained), often containing microscopic crystals of plagioclase feldspar and pyroxene; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-glassy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-glassy; Crystal Structure: Aphanitic (fine-grained), often containing microscopic crystals of plagioclase feldspar and pyroxene; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been chemically and mechanically weathered into a smooth pebble shape by river or coastal water movement. Most surface basalt dates from the Cenozoic era, though it has formed throughout Earth's history.
Uses & applications
Crushed basalt is used in construction as aggregate for roads, railroad ballast, and concrete. In polished or natural pebble form, it is used in landscaping, river rock decor, and hot stone massage therapy.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars. When basalt cools slowly in thick layers, it can form unique hexagonal columns, such as the Giant's Causeway.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It will not scratch easily with a steel knife. Commonly found in volcanic regions, riverbeds downstream from volcanic mountains, and along coastal beaches.
